Rectifying devices



March 22, 1966 H s, SQMMERSy JR 3,242,016

RECTIFYING DEVICES Original Filed Jan. '7, 1960 2 Sheets-Sheet 11 Z0 f4 if /7 Y 7m/MM.

March 22, 1966 H. s. SOMMERS, JR

RECTIFYING DEVICES 2 Sheets-Sheet 2 Original Filed Jan. '7, 1960 aw k. a f MW W /f/ m $0 me w r/ 5 M E E fr I, /I Z VM ,C 0 j NM 4 I j .w w n .o 35 l .af 5 mm F YM/ JM HE W 1./ 0 w. M N 7 W 6v //df//0 P/fw M M m 6 4 z a 4. T n mb, W QEE n m a 0 m 00 United States Patent O 3,242,016 RECTIFYING DEVICES Henry S. Sommers, Jr., Princeton, NJ., assigner to Radio Corporation of America, a corporation of Delaware Original application Jan. '7, 1960, Ser. No. L019, new Patent No. 3,119,072, dated Jan. 21, 1964. Divided and this application Get. t8, i963, Ser. No. 317,252

2 Claims. (Cl. 14S-33.1)

This invention which relates to electrical wave rectifying devices is a division of co-pending application Serial No. 1,019, filed January 7, 1960, now Patent No. 3,119,072, entitled Rectifying Circuits, and assigned to Radio Corporation of America.

Known types of diode rectifiers used in rectifying or detecting circuits exhibit a relatively low conductance for reverse bias voltages, and a relatively high conductance for forward bias voltages beyond a predetermined threshold, which may, for example,`be on the order of 350 millivolts (mv.). The forward bias voltage at which the transition from the low conductance condition to the high conductance condition occurs in known types of diodes varies with changes in temperature. For this reason, such diodes `are operated with `Zero bias when used in rectifying or detecting circuits and, therefore, are not suitable for use in low signal level detector circuits since the detection efficiency is extremely poor unless the amplitude of an applied signal is sufiicient to drive the diode well into its high conductance condition.

A voltage controlled negative resistance diode, such as a tunnel diode, may be used as the nonlinear element of a rectifying or detecting circuit. For reverse bias voltages, a tunnel diode exhibits a high conductance. For forward bias voltages of increasing value, the tunnel diode first exhibits a high conductance which then decreases sharply to first zero conductance condition at a relatively small voltage. As the forward bias voltage is increased still further, the tunnel diode exhibits a negative conductance which first increases and then decreases to a second zero conductance condition. At still higher forward bias voltages the tunnel diode exhibits a positive conductance that increases to a high value in about the sarne manner and at about the same bias voltage as known types of junction diodes. It has been found that the forward voltage at which the first zero conductance condition occurs is not appreciably affected by temperature. However, the transition voltage range from the second zero conductance condition to the second high conductance condition varies with temperature in about the same manner as known types of diodes.

As used herein the term forward bias voltage indicates that the layer of material with p type impurities is biased positively with respect to the layer of material containing n type impurities, and the term reverse bias indicates that the layer of material containing p type impurities is biased negatively with respect to the layer of material containing n type impurities.

For small signal voltage swings, the average conductance of the tunnel diode for excursions in the forward bias direction toward, into or through a portion of the negative conductance region, is much less than for excursions in the reverse direction. Thus the detector circuit including a tunnel diode provides relatively high detection or rectification efficiency for small signals as compared to the relatively low detection efiiciency of known types of diode detector circuits for signals of the same low level. Furthermore, a tunnel diode detector subject to temperature variations may be biased for stable operation at a point near the first zero conductance condition to further improve the efficiency of detection for small signals.

3,Z452,l Patented Mar. 22, i966 The polarity of the direct current (D.C.) component developed across a load impedance element of a tunnel diode detector circuit is opposite to that developed across the load impedance element of known detector circuits for the same anodecathode poling of the diodes. This is because the tunnel diode is more conductive for voltage swings in the reverse direction, and conventional diodes are more conductive for voltage swings in the for ward bias direction. A low level signal detector in accordance with the invention, provides a signal overload characteristic for signals of sufficient amplitude to drive the tunnel diode into the second high forward conductance region. This effectively limits the D.C. output level from the detector circuit to a predetermined maximum value which is substantially independent of the input signal strength for higher amplitude signals.

In accordance with the invention the linearity of detection can be improved by reducing or flattening the negative conductance portion of the tunnel diode characteristic. This may be effected `by providing suitable resistance means in parallel with the tunnel diode. If the ohmic value of the resistance means is approximately equal to the absolute value of the minimum negative resistance of the diode, then the resultant conductance of the combination over at least a portion of the negative conductance region is very low. If desired the parallel or shunt resistance means may comprise a physical resistor, or may be built into the diode. For example, in the construction of a tunnel diode by the dot alloy method, to be described, the diode is etched to clean up the surface only to the extent that the leakage current is reduced to the proper value to provide an equivalent shunt resistance of the desired value. Alternatively, the appropriate shunt resistor may be incorporated in the same mount or capsule with the diode.

The novel features which are considered to be charac teristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation as well as additional advantages and objects thereof will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE l is a sectional view of a typical tunnel diode which may be used in detecting or rectifying circuits;

FIGURE 2 is a graph illustrating the voltage-current characteristics of a tunnel diode of the type shown in FIGURE 1, superimposed on the voltage-current characteristic of a typical junction diode;

FIGURE 3 is a schematic circuit diagram of a halfwave power rectifier circuit including a tunnel diode;

FIGURE 4 is a schematic circuit diagram of a fullwave power rectifier circuit;

FIGURE 5 is a schematic circuit diagram of a voltagedoubler circuit;

FIGURE 6 is a Schematic circuit diagram of a superheterodyne signal receiver including a low level detecting circuit;

FIGURE 7 is a schematic circuit diagram of a tuned low level signal detector circuit;

FIGURE 8 is a schematic circuit diagram of a modification of the tuned low level signal detector shown in FIGURE 7;

FIGURE 9 is a graph of a tunnel diode voltage-current characteristic as modified by a series connected resistor;

FIGURE l0 is a schematic circuit diagram of a modiication -of a low level signal detector;

FIGURE ll is a schematic circuit -diagram of another embodiment tunnel diode detector; and

FIGURE l2 is a schematic circuit diagram of still another embodiment of a tunnel diode detector.

Reference is now made to FIGURE l which is a diagrammatic sectional view of a typical negative resistance diode. By way of example, Leo Esaki, Physical Review, vol. 109, page 603, 1958, has reported a thin or abrupt junction diode exhibiting a negative resistance over a region of low forward bias voltages, i.e., less than 0.3 volt. The diode which is known as a tunnel diode, was prepared with a semiconductor having a free charge carrier concentration several orders of magnitude higher than that used in conventional diodes.

A diode which was constructed and could be used in practicing the invention includes a single crystal bar of n-type germanium which is doped with arsenic to have a donor concentration of 4.0 1019 cm.-3 by methods known in the semiconductor art. This may be accomplished, for example, by pulling a crystal from molten germanium containing the requisite concentration of arsenic. A wafer is cut from the bar along the 111 plane, i.e. a plane perpendicular to the 111 crystallographic axis of the crystal. The wafer 10 is etched to a thickness of about 2 mils with a conventional etch solution. A major surface of this wafer 10 is soldered to a strip 12 of a conductor, such as nickel, with a conventional lead-tin-arsenic solder, to provide a non-rectifying contact between the wafer 10 and the strip 12. The nickel strip 12 serves eventually as a base lead. A 5 mil diameter dot 14 of 99 percent by weight indium, 0.5 percent by weight zinc and 0.5 weight percent gallium is placed with a small amount of a commercial flux on the free surface 16 of the germanium wafer 10 and then heated to a tempera-ture in the neighborhood of 450 C. for one minute in an atmosphere of dry hydrogen to alloy a portion `of the dot to the free surface 16 of the wafer 10, and then cool-ed rapidly. In the alloying step, the unit is heated and cooled as rapidly as possible so as to produce an abrupt p-n junction. The unit is then given a final d ip etch for 5 seconds in a slow iodide etch solution, followed by rinsing in distilled water. The etching step cleans up the surface of the wafer around the dot to reduce leakage current between the wafer and the dot. A suitable slow iodide etch is prepared lby mixing one drop of a solution comprising 0.55 gram potassium iodide, and l0() cm.3 water in l0 cm.3 concentrated acetic acid, and 100 cm.3 concentrated hydroiiuoric acid. A pigtail connection may be soldered to the dot where the device is to be used at ordinary frequencies. Where the device is to be used at `high frequencies, contact may be made to the dot using low impedance encapsulation techniques.

Other semiconductors may be used instead of germanium, particularly silicon and the IILV compounds. A III-V compound is a compound composed of an element from Group III and an element from Group V of the Periodic Table of Chemical Elements, such as gallium arsenide, indium arsenide and indium antimonide. Where III-V compounds are used, the p and n type impurities ordinarily used in those compounds are also used to form the diode described. Thus, sulfur is a suitable n-type impurity and zinc a suitable p-type impurity which is also suitable for alloying.

The voltage-current characteristic of a typical diode is shown by the curve 17 of FIGURE 2. The current scales depend on area and doping of the junction, but representative currents are in the milliampere range.

For a small voltage in the back or reverse direction, the back current of the diode increases as a function of voltages as is indicated by the region b of the curve 17. This indicates a high conductance in the reverse direction. For small forward bias voltages, the forward current increases as a function of voltage (curve 17, region c). The forward current results due to quantum mechanical tunneling. At higher forward bias voltages, the forward current due to tunneling reaches a maximum (region d, curve 17), and then begins to decrease. This drop continues (region e) until a current minimum is reached (region f) and eventually normally injection over the barrier becomes important and the characteristic turns into the usual forward behavior (region g). Translated into terms of conductance, the regions b, c and g exhibit high positive conductance, the region e exhibits a negative conductance, and the regions d and f exhibit low conductance.

The voltage-current characteristic of known types of junction diodes is contrasted with that of the tunnel diode in the curve 18 of FIGURE 2. The conventional junction diode exhibits a low conductance over a range of reverse bias voltages as shown in the region h out to the breakdown voltage of the diode, not shown. For forward bias voltages the diode exhibits a low conductance, as shown in the region j, out to a threshold value, and a high conductance for greater magnitudes of forward bias voltages as shown in region Ic.

The high conductance characteristic in the region k of conventional diodes and in region g for tunnel diodes is temperature sensitive. In other words, the voltage at which the high conductance characteristics indicated by the regions g and k of the curves 17 and 1SA respectively occurs, varies with temperature. On the other hand, the voltage at which the current maximum (region d) of the tunnel diode characteristic occurs `has been found to be substantially unaffected by temperature.

A half-wave power rectifier circuit is shown in FIG-l URE 3. The rectifier includes a power transformer 2,0 having a primary winding 22 and a secondary winding 23. A tunnel diode 24, a filter inductor 25 and a load resistor 26 are connected in series across the secondary winding 23, and any voltage developed across the resistor 26 may be applied to a suitable load circuit, not shown, by way of terminals 27 and 28.

The circuit shown in FIGURE 3 is particularly useful for supplying a low value of direct bias voltage at low impedance such as is required of the bias voltage supply source for certain tunnel diode circuits. In a practical example of the circuit, power voltage such as volts, 60 cycles is applied to the primary winding 22. The turns ratio between the primary winding 2,2 and secondary winding 23 is such that a small voltage such as on the order of 2 volts peak-to-peak is developed across the secondary winding 23. Since the average conductance of the tunnel diode 24 for alternating voltage excursions in the forward direction is less than in the reverse direction, a direct voltage component will be developed across the resistor 26 with the terminal 27 being positive with respect to the terminal 28. The inductor 25 serves as a filter to reduce the ripple component of the voltage developed across the load resistor 26.

If desired, the half-wave rectifier circuit of FIGURE 3 may be modified to provide full-wave rectification as shown in FIGURE 4. As shown in FIGURE 4, a power transformer 30 having a primary winding 31 and a center tapped secondary winding 32 drops the value of the alternating line voltage applied to the primary winding 31 to a nominal value such as 2 volts peak-to-peak across each half of the secondary winding 32. A tunnel diode 33 and a filter inductor 34 are connected in series with a load resistor 35 across one half of the secondary winding 32, and a tunnel diode 36 and a filter inductor 37 are connected in series with the load resistor 35 across the other half of the secondary winding 32. The circuit across each half of the secondary winding 32 operates in the same manner as the half wave rectifier as described above in connection with FIGURE 3 except that the greater conductivity of the rectiliers 33 and 36 occurs on opposite half cycles of the applied voltage wave. A D.C. output voltage for application to suitable load may be derived from the output terminals 38 and 39, with the terminal 38 being positive with respect to the terminal 39.

The application of tunnel diodes to voltage doubler circuits is shown in FIGURE 5. An alternating current wave from a suitable source, not shown, is applied be,`

tween a pair of input terminals 40 and 41. A capacitor 42 and a tunnel diode 43 are connected in series between the input terminals. A tunnel diode 44 and a capacitor 45 are connected across the tunnel diode 43, and a load resistor 46 is connected across the capacitor 45.

Since the tunnel diodes 43 and 44 exhibit a greater average conductance for reverse bias voltages than for forward bias voltages, the portion of the cycle wherein the input terminal 40 is driven negative by the applied alternating current wave produces a greater current fiow through the diode 43 than through the diode 44. This causes the capacitor 42 to change so that the plate thereof connected to the diodes is more positive than the plate connected to the terminal 40. n the succeeding half cycle wherein the terminal 40 is positive with respect to the terminal 41, the average conductance of the diode 44 is greater than that of the diode 43. The positive going portion of the wave applied to the terminal 40 adds to the charge on the capacitor 42 to cause increased current through the diode 44, causing a greater direct Voltage to be developed across the capacitor 4S than would be developed by half wave or full wave rectification. The D.C. Voltage which appears across the load resistor 46 may be applied from the output terminals 47 and 48 to a suitable load circuit, not shown.

The circuits shown in FIGURES 3, 4 and 5 incorporating tunnel diodes, exhibit a higher rectification efficiency for very low applied voltages than is achieved with conventional diodes for low values of applied voltages. Since the tunnel diode may be made to present a very low intrinsic resistance, this permits the design of rectifier circuit to provide relatively low D.C. voltages at low impedance levels.

FIGURE 6 is a schematic circuit diagram partly in block form of a superheterodyne signal receiver. Radio frequency wave energy intercepted by the antenna 50 is applied to the input circuits 52 of the receiver which includes the signal selection or tuner circuits, radio frequency amplifying circuits, mixer circuits, and oscillator circuits all of which are well known in the art. The received radio frequency wave is heterodyned to a corresponding signal modulated wave of intermediate frequency (I-F) by the circuits 52 and applied to an I-F amplifier 53 for further fixed frequency amplification. The I-F amplifier 53 output circuit includes an output transformer 54 which drives a low level detector circuit embodying the invention. The I-F transformer 54 includes a secondary winding 55 across which is connected the parallel combination of a tunnel diode 56 and a resistor 57 in series with the parallel combination of a capacitor 58 and a resistor 59 having .an adjustable tap. Audio frequency waves developed across the output resistor 59 are applied to an audio amplifier 60 which drives a loudspeaker 61.

The resistor 57 connected in parallel with the tunnel diode 56 modifies the tunnel diode characteristic to improve the linearity of detection thereof. Although the resistor 7 is shown as a physical resistor in the schematic circuit diagram of FIGURE 6 it is to be understood that this resistor may represent the equivalent resistance across the junction of the tunnel diode which could be produced, for example by controlling the slow iodide etch step so that sufficient leakage current remains to produce the equivalent of the resistor 57.

The curve 49 of FIGURE 2 shows the voltage-current characteristic of a tunnel diode having a shunt resistance that is slightly less than the absolute value of the minimum negative resistance that the diode would exhibit in the absence of the shunt resistance. It will be noted by comparison of the curves 17 and 49 that the major alteration of the diode voltagecurrent characteristic by the parallel resistor 57 is that the negative slope, region e of the curve 17, is flattened, and the negative resistance effect of the diode 56 is eliminated or is slightly positive. The region e of the curve 49 shown in FIG- URE 2 thus exhibits a substantially constant low conductance. The region b of the curve 49 is similar to the region b of the curve 17, and exhibits about the same high conductance characteristic as illustrated in the drawmg.

Intermediate frequency signals of extremely small magnitude that are applied to the tunnel diode 56 are bypassed around the load resistor 59 through the capacitor 58. Signals applied to the diode 56 are detected by virtue of the fact that the tunnel diode exhibits a lower conductance for signal swings in the forward bias direction than for signal swings in the reverse bias direction. The detected signals are developed across the resistor 59 and applied to the audio amplifier 60. For small signals the detection efiiciency of circuits using a Zero biased tunnel diode is much greater than that of circuits using known types of junction diodes. Detection eficiency -may be regarded as the ratio of the audio power output to the I-F power input. The magnitude of the D.C. Voltage component developed across the resistor 59 is roughly proportional to the extent that the applied signal drives the zero biased tunnel diode in the forward direction pas-t the knee of the curve 49 (region d) whereas in a conventional diode no appreciable direct voltage is produced until the applied signal voltage is of sufiicient magnitude to drive the diode significantly into the region g. In other words, to achieve the same detection efiiciency, the amplitude of the signal voltage applied to conventional diode detector circuits mus-t be several times as great as that applied to the tunnel diode detectorl circuits. Accordingly, the amount of amplification ahead of the second detector is materially reduced for receivers using tunnel diode detectors. Low level tunnel diode detector circuits wherein the tunnel diode is not parallel by resistance means may be used if desired, at a sacrifice lin the linearity of detection.

Another aspect of the operation of tunnel diode detector circuits is that for signals above a certain level, the D.C. output voltage is substantially independent of the applied high frequency voltage. This limiting action occurs when the applied signal level becomes large enough to drive the diode into the secon-d forward bias high conductance region g or g of the curves 17 and 49 respectively. In this regard, the tunnel diode provides a high conductance for the peaks of large amplitude input signals or both positive and negative half cycles, and accordingly no further change in D.C. output voltage will occur, except the small change caused by the differences in the forward and reverse high conductance characteristics (regions b' and g).

For detection at a fixed frequency a low level detector circuit of the type shown in the schematic circuit diagram of FIGURE 7 may be used. The circuit of FIGURE 7 is substantially the sa-me as that shown in FIGURE 6 with the addition of an inductor 62 which is connected in parallel with the tunnel diode 56. For high frequency signals the capacitive reactance of the tunnel diode may become quite low, and effectively bypass the signals around fthe diode. The inductor 62 is selected to resonate with `the tunnel diode interelectrode capacitance .at the midband frequency of the signals applied thereto. The resultant parallel resonant circuit provides a high series impedance to the applied signals, and neutralizes fthe shunting effect of the tunnel diode capacitance. When the tunnel diode 56 is driven in the reverse bias direction by the applied signals, the conductance thereof is so high that the high impedance of the tuned circuit can be neglected. When the diode 56 is driven in the forward direction by the applied signals, the average conductance of the diode and tuned circuit may be maintained at a relatively low value.

FIGURE 8 is a schematic circuit diagram of a low level detector circuit similar to that shown in FIGURE 7 with the exception that a resistor 63 has been added in series with the tunnel diode 56". The resistor 63 provides the detector circuit with a flatter overload detection characteristic for larger forward and reverse bias voltages. The effect of the series resistor 6?; on the conductance characteristic of the tunnel diode 56" is shown in a graph of FIGURE 9. It will be noted that the resistor 63 Vtends to reduce the slope of the regions g" and b" and make them parallel. This modifies the overload or limiting characteristic of the detector circuit so that higher levels of applied signals may be accommodated without overloading the following stages. The series resistor 63 can be built into the diode structure either by increasing the base resistance through grading the doping or by adding a series resistor inside the mount.

FIGURE 10 is a schematic circuit diagram of a low level detector similar to that shown in FIGURE 6 with the exception that a D..C. biasing voltage is applied to the diode 56 with a suitable battery 64 and an R-F choke 65. The battery is preferably selected to bias the diode close to the knee of the curve 49 (region d)2 to further improve the eiiiciency lof low level signal detection and still further reduce the lower limit for linear detection. The application of bias to a tunnel diode detector is practical sinee the voltage at which the knee of the curve i9 (region cl) occurs is substantially unaffected by temperature.

A resistor 70 is connected in parallel with the tunnel diode 56 across which is developed the necessary diode biasing voltage. The resistance value of the resistor 70 is selected to provide the desired voltage-current characteristics of the diode, such as the characteritsic represented by the curve 49. of FIGURE 2. If the bias voltage supplied to the diode is in the range corresponding to the region e of the curve 17, then the ohmic value of the resistor 70 must be less than the absolute value of the maximum nega-tive resistance of the diode to prevent switching.

Biasing voltage for the diode may be applied directly from a low resistance voltage source, shown as an R-F choke coil 65 and a battery 64 in FIGURE 1 1. A small resistor 66 with resistance less than the absolute value of the negative resistance of the diode may be added to prevent surging of the choke coil 65. When biased to the region. d 0f the Curve 17 of. FIGURE 2, the circuit Of FIGURE 11 provides f ull wave reetiiication of the applied signal wave. This may be understood by observing that both half c ycles of the applied signal wave cause a reduction in current through the tunnel diode.

As shown in FIGURE 12, the tunnel diode detector circuit may be simplified by deriving the detected signal from a resistor 72 connected across the terminals of the diode. This circuit not only has the advantage of eliminating the additional load resistor, but also eliminates the loss in power dissipated by the shunt resistor.

What is claimed is:

1. A unitary circuit element comprising a tunnel diode including a pair of semiconductor regions and a tunnel junction therebetween, and a resistance structurally integrated with and forming a part of said regions providing a resistive leakage path between said regions,

Vsaid element exhibiting a current-voltage characteristic including a first portion in which the current increases in response to increased reverse bias Voltage,

a second portion in which the current increases in response to increased forward bias Voltage of relatively small values,

a third portion in which the current is substantially cons-tant in response to increased forward bias voltage of inter-mediate values, and

a fourth portion in which the current increases in respense to increased values of forward bias voltage of relatively large values.

2. A unitary circuit element comprising a tunnel diode including a pair of semiconductor regions and a tunnel junction therebetween, and a resistance consisting essentially of a disturbed surface structurally integrated with and forming a part of said regions providing a resistive leakage path between said regions,

the current-voltage characteristic of said path being matched to that of said diode to impart to said ele` ment a composite current-voltage characteristic including a first portion in which the current increases in response to increased reverse bias voltage,

a second portion in which the current increases in response to increased forward bias voltage of relatively small values,

a third portion in which the current is substantially constant in response to increased forward bias voltag-e of intermediate values, and

a fourth portion in which the current increases in response to increased values of forward bias voltage of relatively large values.

References Cited by the Examiner UNITED STATES PATENTS 2/1957 Mueller 317-234 2/1963 Rutz 307--885 

1. A UNITARY CIRCUIT ELEMENT COMPRISING A TUNNEL DIODE INCLUDING A PAIR OF SEMICONDUCTOR REGIONS AND A TUNNEL JUNCTION THEREBETWEEN, AND A RESISTANCE STRUCTURALLY INTEGRATED WITH AND FORMING A PART OF SAID REGIONS PROVIDING A RESISTIVE LEAKAGE PATH BETWEEN SAID REGIONS, SAID ELEMENT EXHIBITING A CURRENT-VOLTAGE CHARACTERISTIC INCLUDING A FIRST PORTION IN WHICH THE CURRENT INCREASES IN RESPONSE TO INCREASED REVERSE BIAS VOLTAGE, A SECOND PORTION IN WHICH THE CURRENT INCREASES IN RE- 